Nanofiltration has been used for separating sugars, such as monosaccharides and disaccharides from each other and from other substances, such as higher saccharides. For example monosaccharides like glucose, fructose and xylose and disaccharides like sucrose have been separated and recovered by various nanofiltration processes. The starting mixtures including monosaccharides, disaccharides and higher saccharides may be of different origin, such as plant-based biomass hydrolysates or starch hydrolysates, for example.
U.S. Pat. No. 5,869,297, Archer Daniels Midland Co. (published Feb. 9, 1999) discloses a nanofiltration process for making dextrose. This process comprises nanofiltering a dextrose composition including as impurities higher saccharides, such as disaccharides and trisaccharides. A dextrose composition having a solids content of at least 99% dextrose is obtained. Crosslinked aromatic polyamide membranes have been used as nanofiltration membranes. Furthermore, it is proposed that a portion of the nanofiltration retentate may be recycled to the nanofiltration feed tank.
WO 99/28490, Novo Nordisk AS (published Jun. 10, 1999) (=U.S. Pat. No. 6,329,182) discloses a method for enzymatic reaction of saccharides and for nanofiltration of the enzymatically treated saccharide solution including monosaccharides, disaccharides, trisaccharides and higher saccharides. Monosaccharides are obtained in the nanofiltration permeate, while an oligosaccharide syrup containing disaccharides and higher saccharides is obtained in the retentate. The retentate including the disaccharides and higher saccharides is recovered. A thin film composite polysulfone membrane having a cut-off size less than 100 g/mol has been used as the nanofiltration membrane, for example. It is recited that the permeate resulting from the nanofiltration may be recycled to the enzymatic reaction.
U.S. Pat. No. 4,511,654, UOP Inc. (published Apr. 16, 1985) relates to a process for the production of a high glucose or maltose syrup by treating a glucose/maltose-containing feedstock with an enzyme selected from amyloglucosidase and β-amylase to form a partially hydrolyzed reaction mixture, passing the resultant partially hydrolyzed reaction mixture through an ultrafiltration membrane to form a retentate and a permeate, recycling the retentate to the enzyme treatment stage, and recovering the permeate including the high glucose or maltose syrup.
U.S. Pat. No. 6,126,754, Roquette Freres (published Oct. 3, 2000) relates to a process for the manufacture of a starch hydrolysate with a high dextrose content. In this process, a starch milk is subjected to enzymatic treatment to obtain a raw saccharified hydrolysate. The hydrolysate thus obtained is then subjected to nanofiltering to collect as the nanofiltration permeate the desired starch hydrolysate with a high dextrose content. Furthermore, it is proposed that at least part of the nanofiltration retentante may be subjected to saccharification to obtain a saccharified nanofiltration retentate. The saccharified nanofiltration retentate may be subjected to molecular sieving, for example by chromatographic separation or nanofiltration, to collect a fraction with a higher dextrose content and a fraction with a lower dextrose content. The dextrose-enriched fraction may then be mixed with the starch hydrolyzate having a high dextrose content obtained previously in the process.
U.S. Pat. No. 6,406,546 B1, Tate & Lyle Industries (published Jun. 18, 2002) discloses a process of obtaining sucrose from a sucrose-containing syrup by nanofiltering the syrup through a nanofiltration membrane and recovering the nanofiltration retentate enriched in sucrose. It is recited that invert sugars are passed through the nanofiltration membrane into the nanofiltration permeate. FIG. 3 of the publication discloses a three-stage nanofiltration process for obtaining a sucrose-containing nanofiltration retentate and nanofiltration permeate. The nanofiltration retentate including the desired product from the three-stage nanofiltration is collected in one fraction, i.e. the retentate from the preceding nanofiltration stage is introduced into the next nanofiltration stage, and the retentate from the last nanofiltration stage is collected. Also the nanofiltration permeate is collected in one fraction.
U.S. Pat. No. 6,406,547 B1, Tate & Lyle Industries (published Jun. 18, 2002) discloses a process for producing sugar (sucrose) from beets by a multistep process, which comprises two successive ultrafiltration steps and a nanofiltration step. The nanofiltration retentate thus obtained and having a high concentration of sucrose can be used in evaporation and crystallization operations to produce crystals of white sugar. The process can optionally include ion exchange and/or electrodialysis purification steps, prior to or after the nanofiltration step. It is also recited that recycle syryps, for example mother liquors from the crystallization can be treated with an enzyme or chromatographic separation to remove raffinose.
US 2003/0092136A1, D. Delobeau (published May 15, 2003) discloses a process for the manufacture of a starch hydrolysate having a high content of dextrose by a two-stage nanofiltration process. A nanofiltration permeate enriched in dextrose (glucose) is recovered. The nanofiltration retentates from both nanofiltration stages may be completely or partially recycled to the nanofiltration feed. The permeate containing the desired product from the two-stage nanofiltration is collected in one fraction, i.e. the permeate from the first nanofiltration stage is introduced into the second nanofiltration stage, and the permeate from the second nanofiltration stage is collected.
US 2002/0079268 A1, J-J Caboche (published Jun. 27, 2002) discloses a process for preparing a fermentation medium for producing high-purity metabolites (such as organic acids, for example optically pure L-lactic acid) from a renewable material (such as wheat solubles or corn steep liquor) by nanofiltration and/or electrodialysis. The purpose of the nanofiltration/electrodialysis is to eliminate low molecular weight impurities from the raw material.
U.S. Pat. No. 5,965,028, Reilly Industries (published Oct. 12, 1999) discloses a process for the separation of citric acid from less desirable components having a molecular weight similar to that of citric acid (such as glucose and/or fructose) by nanofiltration. A nanofiltration permeate enriched in citric acid is recovered. Citric acid is then crystallized from the nanofiltration permeate. A portion of the mother liquid from the crystallization may be recycled to upstream and/or downstream of the nanofiltration step, after a recovery step to recover citric acid. The feed used for the nanofiltration is typically a clarified citric acid fermentation broth.
M. Saska et al. discuss the decolorization of white cane sugar by nanofiltration in “Direct Production of White Cane Sugar with Clarification and Decolorization Membranes”, Sugar Journal, November 1995, pp. 19 to 21 and December 1995, pp. 29 to 31. Decolorization of ultrafiltered clarified juice was carried out with G-10 thin-film nanofiltration membranes (Desalination Systems Inc., Escondido, Calif.) having a molecular weight cut-off of 2500 daltons.
N. Aydogan et al. (Department of Chemical Engineering, Middle East Technical University, Ankara, Turkey) discuss the separation and recovery of sugars by nanofiltration in “Effect of operating parameters on the separation of sugars by nanofiltration”, Separation Science and Technology (1998), 33(12), pp. 1767-1785. For example, it was found that with an increase of the feed flow rate, permeate flux increased. It was also found that there is a linear relationship between the pressure and the permeate flux up to 30 bars. To investigate the effect of the concentration, 1 to 10 weight-% solutions of sucrose and glucose were utilized, whereby it was found that with an increase in the concentration, permeate flux decreased.
M. L. Bruening et al. (Department of Chemistry, Michigan State University, East Lansing, Mich. USA) have investigated the behaviour of multilayer polyelectrolyte membranes in “Nanofiltration with multilayer polyelectrolyte membranes”, PMSE Preprints (2003), 89, 169. It is recited that minimum thickness of the polyelectrolyte films as nanofiltration membranes affords high flux in the nanofiltration. Furthermore, it was found that the charge was the primary factor in the nanofiltration of small neutral molecules (such as methanol and glycerol). It is also recited that a selectivity of more than 100 between larger neutral molecules (i.e. between glucose and sucrose) was achieved.
Chemistry and Industry of Forest Products, vol. 22, No. 1, 2002, pp. 77-81 discloses a review discussing the application of membrane separation in desalinization, concentration and purification of xylan extracts, separation of xylo-oligosaccharides from xylan hydrolysates, and the classification and purification of oligosaccharides. Examples of processing renewable plant resources using membrane separation are given. These include, for example, continuous ethanol fermentation coupled with membrane separation and the concentration of plant xylose solution by nanofiltration.
G. Yang et al. (Membrane Science and Technology Research Center, Nanjing University of Chemical Technology, Nanjing, China) discuss the nanofiltration of xylose in “Concentration of xylose solution through nanofiltration”, Mo Kexue Yu Jishu (2000), 20(5), 21-26 (Journal written in Chinese). In this study, two types of spirally wound nanofiltration modules differing in the cut-off size were used to study the nanofiltration process of crude industrial xylose. It is recited that the xylose solution was concentrated from 4% to 20% in the nanofiltration retentate by a nanofiltration equipment comprising a 4-stage serial connection configuration.
G. S. Murthy et al. (Membrane Separations Group, Chemical Engineering Division, Indian Institute of Chemical Technology, Hyderabad, India) discuss the concentration of xylose by nanofiltration in “Concentration of xylose reaction liquor by nanofiltration for the production of xylitol sugar alcohol”, Separation and Purification Technology 44 (2005) 221-228. Pilot scale nanofiltration experiments were carried out using a polyamide (PA) spiral membrane module having 300 molecular weight cut-off and 1 m2 effective area. The concentrate (reject) flow rate was fixed and continuously recirculated to the feed tank through a heat exchanger. It is recited that at a feed pressure of 20 bar, xylose was concentrated from 2 to 10% at a reasonably high average flux of 241/l(m2h) and rejection of >99% which indicated negligible losses of the sugar in the permeate. The feed for the nanofiltration was an acid hydrolysate of rice husk. In accordance with this reference, xylose is concentrated in the nanofiltration retentate. The purity of concentrated xylose product in relation to the other components of the rice husk hydrolysate is not discussed.
WO 02/053783, Danisco Sweeteners Oy (published 11 Jul. 2002) discloses a process of producing a xylose solution from a biomass hydrolysate by subjecting the biomass hydrolysate to nanofiltration and recovering as the nanofiltration permeate a solution enriched in xylose. The aim in the process is to concentrate xylose in the nanofiltration permeate and to collect the total permeate in one permeate fraction. The feed used for the nanofiltration may be for example a spent sulphite pulping liquor containing a mixture of other closely-related monosaccharides, such as glucose, galactose, rhamnose, arabinose and mannose, in addition to the desired xylose. It was found that the nanofiltration effectively concentrated pentose sugars, such as xylose in the nanofiltration permeate, while hexose sugars remained in the nanofiltration retentate. However, the permeate obtained from the nanofiltration had a relatively low dry substance content (1 to 2%) and consequently a low xylose content. Furthermore, the xylose yields were low (less than 20%). Hereby the performance of the process was not sufficient for efficient industrial operation.
WO 02/053781, Danisco Sweeteners Oy (published 11 Jul. 2002) discloses a nanofiltration process of separating small molecular compounds from each other by nanofiltration, whereby the difference of the molar masses of the compounds to be separated is less than 1.9-fold, such as pentose sugars (xylose and arabinose) from hexose sugars (glucose, galactose, rhamnose, mannose), xylitol from sorbitol, betaine from erythritol as well as betaine from glucose and inositol. The aim in the nanofiltration is to concentrate the product in the nanofiltration permeate/retentate and to collect the total permeate/retentate in one permeate/retentate fraction.
WO 02/053783 and WO 02/053781 (Danisco Sweeteners) discussed above also disclose for example the chromatographic fractionation as a pretreatment step for the nanofiltration process as well as a test arrangement of recycling the permeate and/or retentate back to the feed vessel without product recovery (total recycling mode filtration).
US 2002/0158021 A (U.S. Pat. No. 6,692,577), Danisco Sweeterners Oy (published Oct. 31, 2002) discloses a process for purifying a maltose-containing liquor from undesired impurities, such as maltotriose. The process comprises nanofiltering the maltose-containing liquor and recovering a purified maltose solution as the nanofiltration permeate.
WO 2004/002938 A1, Finnfeeds Finland Oy (published 8 Jan. 2004), discloses a process for recovering betaine and sucrose by a combination of nanofiltration and chromatography from a sugar beet-derived solution, such as molasses solution. The nanofiltration and chromatographic fractionation steps may be carried out successively in any desired sequence. The reference also teaches that the nanofiltration permeate as it is obtained from the nanofiltration may be returned to the chromatographic fractionation to be used therein as the eluent.
WO 2004/003236 A1, Danisco Sweeteners Oy (published 8 Jan. 2004) discloses a nanofiltration process of removing crystallization inhibitors from sugars. In a typical embodiment of the process, the sugars are selected from monosaccharide sugars, such as xylose and fructose, and the crystallization inhibitors are selected from disaccharides and higher saccharides.
Nanofiltration of multicomponent mixtures as a rule aims at operating with as high yield as possible, which in general leads to a low performance, a lower flux, slower passing of the nanofiltration solution through the nanofiltration membrane, high membrane surface area demand and low purity of the desired product. All these phenomena accelerate towards the end of the nanofiltration process and together lead to decreased capacity of the nanofiltration process.
One way to solve the problems above is the selection of different membranes. In the present invention, however, the problems above have been solved by collecting the nanofiltration permeate/retentate in several fractions with different purity and recovering the desired component from the collected fractions with favourable processes to improve the performance of the process.